As for Canada, the Great Lakes and the St. Lawrence River are natural borders. In the case of Mexico, that would be the Rio Grande river.
The Great Lakes, St. Lawrence River
The Atlantic Ocean.
The Atlantic Ocean
That would be the Rio Grande river.
Lake Erie
No. The Gulf of California is a body of water on the west of Mexico which separates the Baja California Peninsula (south of the US state of California) from the mainland Mexico. The Gulf of Mexico is on the southeastern US and east of Mexico, and US states such as Florida, Louisiana and Texas have shores on that body of water.
The Atlantic Ocean.
I'm guessing you want the Gulf of Mexico, since that separates the US from Mexico (to some extent, though the Rio Grande does that more thoroughly), Puerto Rico, and other Spanish-speaking islands in the Caribbean. ><><><><><>< Not Mexico, and Puerto Rico is not a country (it is a Commonwealth of the US) But the Gulf does separate the US from Cuba.
None. There is only the man-made border that separates the US from Mexico.
The Northern Atlantic
The Gulf of Mexico.
The Gulf of Mexico.
Mexico and Panama
There are two of them: The Pacific Ocean and the Gulf of Mexico.
The Caribbean Sea